How long have you had them? Have you had lots of rain?
Wet soggy soil is the nemesis of conifers. They don't like to sit in mucky soil for prolonged periods. Are they at the bottom of a slope where water pools? At this time of year, I'd rule out bugs or mites.

Many conifers do naturally lose their bottom branches as they mature. Apparently they can also go brown if you have pruned the tips of the branches.